The present invention generally relates to a piezoelectric resonator and more particularly, to a chip type piezoelectric resonator which utilizes an energy trapped type thickness shear vibration mode of a piezoelectric substrate.
Generally, as shown in FIG. 8, a quadrature detecting circuit 6 is so arranged that an output of a limiter amplifier 1 as the last stage of an FM intermediate frequency amplifier coupled therewith at a terminal 8 is applied to a phase comparator 2 as one input signal (reference signal), while a signal obtained by shifting the phase of the output signal of the limiter amplifier 1 through approximately 90.degree. at a center frequency of the FM intermediate frequency by a phase shifter 4 constituted by a phase advancing capacitor C0 coupled at a terminal 7 to a tank circuit 3 including a coil L1 with a variable inductance and a capacitor C1 connected parallel to each other, is employed as the other input signal to said phase comparator 2 so as to be converted by said phase comparator 2, into a pulse train signal whose pulse width is varied corresponding to the amount of variation of both of the above input signals for being fed into a final stage amplifier 5, and thereafter, the pulse train signal is passed through an integration circuit (not shown), thereby to obtain its average value as an FM demodulation signal at a terminal 9. In the quadrature detecting circuit as referred to above, it has been attempted to make adjustments of the phase shifter 4 unnecessary by replacing the coil L1 and capacitor C1 in the tank circuit 3 with a two-terminal type piezoelectric resonator 11 and a damping resistance R0 for damping impedance in the vicinity of an anti-resonance frequency of said resonator 11 as in a modified tank circuit 3' in FIG. 9.
Conventionally, in the quadrature detecting circuit 6 as shown in FIG. 9, there has been employed for the two-terminal type piezoelectric resonator 11, a piezoelectric resonator as shown in FIG. 10, which includes a piezoelectric substrate 21 of a high mechanical factor Qm, electrode layers 22 and 23 formed on opposite main surfaces of said substrate 21, an energy trapped type thickness shear vibration mode portion 24 of the substrate 21 held between confronting end portions of the electrode layers 22 and 23 and subjected to polarization as shown by an arrow P, and a covering layer 25 made, for example, of a resilient material such as silicone rubber, etc. and applied onto the outer periphery of said portion 24, so that the energy trapped type thickness shear vibration of the piezoelectric substrate 21 is damped by the covering layer 25 for lowering the mechanical quality factor Qm of said two-terminal type piezoelectric resonator 11, thereby to enlarge a demodulating region band width of the quadrature detecting circuit, and also, to improve output frequency characteristic and distortion factor characteristic thereof.
However, in the two-terminal type piezoelectric resonator 11 as shown in FIG. 10, although the covering layer 25 of silicone rubber and the like is formed on the outer peripheral portion of the energy trapped type thickness shear vibrating portion 24 of the substrate 21, for example, by painting, etc., it is difficult to suppress scattering or deviation in the shape of said covering layers 25 upon curing, and when the outer peripheries of the piezoelectric substrates 21 are covered by the electrically insulative resin layers, such scattering in the configuration of the covering layers 25 is noticed as it is, also in the finished products thus clad.
When the two-terminal type piezoelectric resonators 11 having the scattering in the external dimensions as described above are actually attached at a high density, on a printed circuit board as piezoelectric resonators for an FM discriminator together with other electronic parts, there has been such a problem that the two-terminal piezoelectric resonators undesirably contact other electronic parts, and thus, can not be properly mounted on the printed circuit board.